1. Technical Field
The present invention relates generally to an apparatus and method for visible light communication and, more particularly, to an apparatus and method for visible light communication that have improved reception performance regardless of dimming control of lighting based on VPPM signals.
2. Description of the Related Art
A visible light communication system that uses lighting such as LEDs or the like is a system for transmitting and receiving information based on lighting that emits visible light. Therefore, wireless communication must be performed under the condition in which basic functions of the lighting are still satisfied. Because one of the important functions of lighting is to control the brightness of the lighting, that is, a dimming function, the visible light communication system must have not only a wireless communication function using visible light but also a dimming function. In connection with visible light communication, techniques for controlling the brightness of lighting include an amplitude dimming method and a Variable Pulse Position Modulation (VPPM) dimming method. The amplitude dimming method adjusts the brightness of a light source by varying the amplitude of a signal in an OOK modulation technique, and the VPPM method provides a brightness control function by changing the width of a pulse.
VPPM is a new modulation method devised from a 2-PPM method and a Pulse Width Modulation (PWM) method in order to prevent the occurrence of intra-frame flickering and to control the brightness of a light source, and is one of the modulation methods for visible light communication adopted in the IEEE 802.15.7 standard. The 2-PPM method represents bit “0” and “1” depending on the position of a pulse as shown in FIG. 1 and provides the same average brightness for both “0” and “1” as being similar to optical power in Manchester code. Therefore, this method may prevent the occurrence of intra-frame flickering. Also, the PWM method controls the brightness of a light source by varying the width of a pulse, and is commonly used in current LED lighting. Meanwhile, the VPPM method is similar to the characteristic of 2-PPM in that it represents bit “0” and “1” depending on the position of a pulse, and is similar to a PWM method in that the width of a pulse may change depending on the brightness desired by a user. Therefore, as shown in FIG. 2, an optical waveform modulated to have a 50% pulse width using the VPPM scheme is the same as a waveform modulated using 2-PPM. Also, FIG. 2 illustrates a mechanism for controlling brightness using a VPPM method, and shows an example in which, when a digital signal “001” is modulated using the VPPM technique, the brightness is changed depending on the variation of the pulse width, even for the same data. Additionally, the VPPM technique divides a level of the pulse width into smaller levels, whereby it is possible to provide maximum brightness similar to that of LED lighting having the same specifications. Also, because the brightness is adjusted depending on the pulse width on the time axis rather than the amplitude of a pulse, a LED light source may be prevented from being damaged and color variation of the light source may be avoided.
Meanwhile, in the digital wireless communication system, a signal may be transmitted by a packet unit or a frame unit. FIG. 3 shows a packet structure generally used in the digital wireless communication system. A preamble is a signal used for detecting a received packet and performing synchronization on the receiver side. In other words, the preamble functions to detect the packet signal in the signals entering the input unit of the receiver and to indicate the start of the packet. After the preamble, a PHY header is located. The PHY header (Physical Layer Fields) contains physical layer information for easily demodulating the modulated transmission data. The physical layer information, generally used in visible light communication, includes a packet length, a channel coding scheme, a line coding scheme, and the like, and these pieces of information are contained in the PHY header and transmitted. Data to be transmitted through the physical layer follows the PHY header, and the data is expressed as Payload in terms of the physical layer.
Recently, with respect to visible light communication, research for improving the performance of reception of VPPM signals is actively being conducted.
Korean Patent Application Publication No. 2010-0060900 discloses a method for enabling visible light communication without impeding the brightness control function of lighting in a lighting device using LEDs, but does not disclose technology for improving the performance of reception of VPPM signals.